Beats me Steve .......... there's usually a thermostatic (or manual) valve at one end (big head with numbers on it), and a "lockshield" valve at the other (used for balancing the rads)
I would have thought her valve would be adjustable........ maybe very old type?
If it is seized up, then it's probably just as well to drain down, flush, and refill, plus additive.
Sometimes, if that's going to be a problem, you could use a freezing kit on that pipe, then change the valve to a new one.
Hang about a bit ........ one of the other guys could be along. They might have come across this before.